Semiconductor superlattices and quantum well heterostructures (QWH's) form an important new class of electronic/optoelectronic materials since the properties of these layered structures are in many ways superior to those of bulk materials. While most superlattices and QWH's are grown using lattice-matched materials, structures comprised of lattice-mismatched heterolayers are also of considerable interest since they offer increased flexibility in choosing constituent materials while exhibiting some interesting properties due to the stress present in the structure.
For example, a heterostructure consisting of biaxially tensile stressed quantum wells can be used to control the light/heavy hole splitting energy and thus modify the effective mass of the charge carriers and the polarization properties of optoelectronic devices fabricated from these stressed heterostructures. Furthermore, changing the compositions and strain layers in these lattice-mismatched material systems permits a designer to selectively adjust the bandgap of the system. One such material system, formed by the growth of InGaAs quantum wells on a GaAs substrate, enables the fabrication of semiconductor lasers which operate in a wavelength range of 0.9 .mu.m to 1.1 .mu.m, a range not easily achieved in lattice matched structures.
Referring specifically to the effect of lattice mismatch in heterolayer structures, it is well known that the stress resulting from this mismatch can be accommodated solely by coherent strain in the heterolayers so long as the thickness of the individual heterolayers is below a critical value. This critical thickness is a function of alloy composition and strain, and is the thickness at which it becomes energetically favorable for misfit dislocations to occur, obviously an undesirable result if the material quality is to be maintained.
At present, conventional research has begun to demonstrate the feasibility of using lattice-mismatched quantum well structures in optical communication applications where large energy bandgap shifts are possible by varying the elastic strain due to lattice mismatches. However, as noted above, the quantum well structures must satisfy a critical thickness constraint that in turn limits the range of allowable bandgap shifts.
This constraint presents a particular problem to the designer who seeks, for example, to decrease the bandgap of a quantum well region to a range suitable for optical communications (1.1-1.3 .mu.m) by increasing the alloy composition x of the quantum well region which, for exemplary purposes, may be an In.sub.x Ga.sub.1-x As single quantum well layer. However, the effect of increasing composition is to correspondingly decrease the critical thickness of the quantum well layers and thereby restrict the designer to the growth of only narrow wells. Unfortunately, narrow wells limit the range of achievable bandgap shifts.
With respect to specific conventional structures, Marioka et al. in U.S. Pat. No. 4,835,583 disclosed a semiconductor device which reduces the influence of lattice mismatch between the substrate and active region by growing a strained superlattice structure therebetween. Another conventional structure, proposed by Gourley et al. in Applied Physics Letters 47(5), 1985, pp. 482-484, attempts to eliminate misfit dislocations in a strained superlattice structure (SLS) epilayer by interposing a thick buffer layer between the SLS and the substrate. As a result, upwardly-propagating defects are confined to the first few layers of the SLS so that the upper layers are nearly free of dislocations.